An important trend in development of semiconductor technology is scaling down of metal-oxide-semiconductor field effect transistors (MOSFETs) for improving integration level and reducing manufacturing cost. However, it is well known that short channel effects arise as the size of MOSFETs decreases.
As the MOSFETs are scaled down, gates also have reduced effective length, so that when a gate voltage is applied, fewer charges may be controlled by the gate voltage in a depletion region. Consequently, threshold voltages of the MOSFETs may drop down with the decreasing of the channel length.
Subthreshold swing (also called S factor) is an important parameter when the MOSFETs operate as logic switches in a subthreshold state during operation. The S factor is defined as: S=dVgs/d(log 10 Id), and the unit is [mV/decade]. S factor has a value equal to a gate voltage increment ΔVgs required to change drain current Id by an order of magnitude. It represents a slope of Id-Vgs curve. The value of S factor depends on device structure and temperature, etc. At the room temperature, the S factor has a theoretical minimum of 60 mV/decade.
As the MOSFETs are scaled down, subthreshold characteristics, which are characterized by S factor, will also degrade. Consequently, a leakage current occurs so that the gate loses control of the drain current.
In order to suppress the short channel effects, an SOI (semiconductor-on-insulator) wafer may be used in manufacturing the MOSFETs. For example, a semiconductor layer of the SOI wafer can have a thickness smaller than 20 nm, which then provides an ultra-thin channel. Hence, the control of charges by the gate voltage in a depletion layer of the ultra-thin channel is enhanced.
However, the ultra-thin channel brings a new technical challenge. The threshold voltage is remarkably changed by thickness variation of the ultra-thin channel. Thus, the thickness of the semiconductor layer in the SOI MOSFETs should be accurately controlled for the desired threshold voltage, which may cause difficulty in manufacturing process.